Clues to Fiery Origin of Life Sought in Hothouse Microbes

By WILLIAM J. BROAD

Published: May 9, 1995

DARWIN pictured life arising in a warm little pond. But scientists are now concluding that the beginning, four billion years ago, was in conditions more like hell and that the progenitor of all terrestrial life was right at home in the searing heat.

Most surprising, experts are finding that today's earthly hot spots teem with organisms that are evolutionary throwbacks to that era. Invisible without a microscope, these tiny specks are now being seen as living relics of the first life, making them as close as science is likely to get to the ultimate ancestor of humans and all creatures.

Once thought of as bizarre oddities, these heat-loving microbes at the outer limits of life have now vaulted to a central scientific role, with the Federal Government devoting more than $10 million to mapping their genes to better understand their workings and evolutionary history. Meanwhile, scientists around the globe are racing to find new ones in terrestrial hot springs and geysers and volcanic vents beneath the sea.

The tiny creatures are known as thermophiles, or heat lovers. In recent years, biologists have discovered them thriving at extraordinarily high temperatures -- up to 235 degrees Fahrenheit, hotter than the usual temperature of boiling water.

Some are bacteria. Some are something stranger. In general, they cannot thrive in the absence of high temperatures, quickly going into a dormant state if things cool down too much.

Remarkably, scientists are finding their genes to be ancient, putting them among the deepest roots of the evolutionary tree.

"At this stage of the game, it's a given that the first life was some kind of thermophile," Dr. Norman R. Pace, a microbiologist at Indiana University who is a pioneer in the study of the unusual microbes, said in an interview.

The origin of life is one of science's most daunting mysteries. Until recently, the conditions under which it arose were a matter of almost pure speculation. Two quite separate lines of inquiry have now edged scientists toward the idea that the earth's first organisms emerged and lived at very high temperatures.

One is the estimate that the surface of the early earth probably remained very hot for many eons into the period when life must have got its first foothold on the planet. The other is the gathering realization that the descendants of this steamy epoch have not only survived but still inhabit special niches reminiscent of their fiery home all over the globe.

Indeed, the heat-loving organisms turn out to be surprisingly common among the many branches of the microbe family, bolstering the idea that they are very old in terms of evolutionary history.

"You find thermophiles in lots of different groups and environments, which suggests that the property of thermophily is a primitive one," said Dr. Thomas D. Brock, a thermophile pioneer at the University of Wisconsin in Madison. "If it were a late adaptation, there'd be no way for it to get around so widely."

The first days of the earth were hellish, with vast chunks of rubble left over from the creation of the solar system slamming into the planet and helping to keep it hot. The main fusillade hit from 4.6 billion to 3.8 billion years ago, the planet's earliest days.

As the molten earth cooled a bit, widespread volcanism began to disperse water vapor and other gases, producing a thick atmosphere and vast oceans. Radioactivity was high. A steady fall of cometary ice, some scientists say, also helped to inundate the world and perhaps sowed organic molecules associated with life. A few scientists speculate that perhaps tiny spores of life from elsewhere in the universe may have fallen to earth to seed the process of biological evolution.

By one alchemy or another, life began, with scientists now putting its start as far back as 4.2 billion years ago, quite early after the earth's formation. Intolerable as were the conditions in this infernal hothouse, it must nonetheless have served as the womb of life, rather than the room-temperature conditions often assumed by early theorists.

Whether indigenous or alien, the first life faced the threat of quick extinction, new research suggests. It appears that the rocky bombardment around four billion years ago was still so violent that the entire planetary ocean was probably vaporized repeatedly.

The thickened atmosphere from such gargantuan jolts would have trapped so much sunlight that the earth's surface temperature became an inferno of more than 3,000 degrees Fahrenheit for thousands of years, sterilizing the earth's surface.

"It seems likely that the result would have been to 'reset the clock' for life's origins," Christopher F. Chyba, Tobias C. Owen and Wing H. Ip, who are planetary scientists, wrote recently in "Hazards Due to Comets and Asteroids," published by the University of Arizona.

As the bombardment slowly let up, temperatures are thought to have settled down, perhaps hovering in the neighborhood of 212 degrees, the temperature of boiling water at sea level. The earth failed to cool off quickly because the high levels of carbon dioxide in the atmosphere kept trapping lots of solar radiation.

By about 3.8 billion years ago, researchers say, the temperature might have dropped to a balmy 120 to 190 degrees.

The earliest probable fossils, dating to about 3.5 billion years ago, have been found by J. William Schopf of the University of California at Los Angeles and other researchers. These appear to be copies of today's cyanobacteria, thermophiles that live comfortably at 160 degrees Fahrenheit or even higher temperatures.

Pasteur announced in 1864 that heat could kill microbes and sterilize a solid or liquid. Pasteurization, widely adopted over the decades, was a boon to public health and food preparation. But exceptions to its usual workings took a century to discover.

In 1964, Dr. Brock, then at Indiana University, visited Yellowstone National Park, home of the celebrated geyser known as Old Faithful. He was looking for uncommon microbes. While sampling hot waters in a woodsy, out-of-the-way area, alert for grizzly bears, he was surprised to find bacteria thriving at temperatures that were thought to be anathema to life.

From a pinkish mass in 1966, he isolated one that flourished at 158 degrees Fahrenheit -- at the time, an unheard-of temperature for life. In 1967, he found microbes living in boiling water. "We kept pushing to higher and higher temperatures," Dr. Brock recalled.

As scientists rushed to global hot spots and uncovered dozens of new kinds of heat-loving microbes, Dr. Carl R. Woese of the University of Illinois in 1977 stunned the scientific world by announcing that some of them constituted a third form of life.

Up to then, scientists divided the living world into two kingdoms according to genetic packaging. The blueprint of life, DNA, floated free in the cells of bacteria, also known as prokaryotes. In the cells of eukaryotes, like fungi, plants and animals, it was gathered into a nucleus.

The DNA of Dr. Woese's new kingdom floated free but was genetically distinct from the bacteria, extravagantly so. "It was a total surprise," he recalled. "The other surprising thing was the resemblance to the eukaryotes. It seemed like a new group in its own right."

He named the new kingdom archaea (pronounced ar-KEY-a), or ancient ones, since their evolutionary history appeared to link them to the earth's earliest life.

Knowledge about the global distribution of such strange microbes jumped after the 1977 discovery in the Pacific's icy depths of hot springs, which were later found to be widely distributed in the deep and to swarm with thermophiles.

It jumped further in the 1980's as Dr. Pace took a clever step to speed their collection from global hot spots. Thermophiles are notoriously hard to culture and grow, with only one species in a thousand or so surviving a laboratory's artificial environment. So Dr. Pace skipped that step. He pioneered the direct analysis of DNA from field samples.

The power of the method was recently driven home by Dr. Susan M. Barns, a colleague of Dr. Pace at Indiana who pulled genetic evidence of more than 40 different types of hot microbes from Yellowstone's Obsidian Pool, which is deep black in color and boiling hot. Preliminary analysis shows that two of its archaea may be among the most primitive organisms on earth.

"It's in the back country," Dr. Barns said of the pool. "There's thousands of hot springs back there."

Of late, scientists have also found thermophiles living deep under the earth itself, where temperatures are uniformly high. Some have been isolated from hot oil reservoirs beneath the North Sea and Alaska's North Slope. Others have been retrieved from depths nearly two miles beneath the continental United States, living at temperatures as high as 167 degrees.

Most important, genetic analyses that highlight the evolutionary relationships among creatures have revealed these heat-loving organisms to be the most primitive on earth, literally living fossils from the earliest days.

The main way of drawing such evolutionary trees is to look at RNA, which contains genetic information copied from the genes. A particularly good kind of RNA to examine is found in the ribosomes that make proteins inside cells. By comparing the ribosomal RNA of different organisms, researchers can discover degrees of evolutionary relatedness.

Based on such analyses involving more than 2,000 organisms, scientists believe that the archaea and bacteria diverged from a common ancestor four billion years ago, soon after life arose. Only later did the forerunners of today's eukaryotes split off from the archaeal branch of the evolutionary tree.

In addition to shedding light on evolution, thermophiles are turning out to be a treasure trove for the biotechnology industry. So great is the overall interest that the Federal Government is supporting four projects to map their DNA. Blueprints for RNA make up less than 1 percent of the genetic material in the typical archaea, so the expansion of the analysis to include the DNA is expected to produced a rush of insights.

No scientist has yet announced the mapping of an organism's complete DNA sequence. But archaea are good candidates because they have relatively small amounts of genetic material, simplifying the analysis. A human cell has about three billion base pairs of DNA. An archaea has about two million.

DNA mapping is now under way on three different kinds of archaea, financed by the Energy and Commerce Departments. Some results are already coming in and the work is to be finished in 1998.

"This initiative will expand our knowledge of how life has evolved and how it sustains itself," Dr. Martha Krebs, a senior official of the Energy Department, said when its Microbial Genome Initiative was announced last year.

Groups now mapping DNA are the University of Utah in Salt Lake City, the Institute for Genomic Research in Gaithersburg, Md., the University of Illinois, the Genome Therapeutics Corporation of Waltham, Mass., and Ohio State University.

Knowledge of the complete DNA sequence will let companies clone bits of it, including the genetic blueprints for enzymes, which are key proteins for performing cellular rearrangements.

Companies that have a mapped some thermophile DNA on their own are racing to isolate, clone and sell the extremely heat-stable enzymes for use in genetic engineering. These enzymes can speed the polymerase chain reaction, or P.C.R., which has spawned a branch of biotechnology that allows scientists to rapidly make trillions of copies of vanishingly small amounts of DNA, including the DNA in fingerprints at crime scenes.

But many scientists say the greatest payoff from the mapping work will be enhanced understanding of the origins of life. Dr. Woese says the federally financed DNA work is already buttressing and fine-tuning the phylogenetic maps based on ribosomal RNA analysis.

Ultimately, he added, such work is likely to to bolster the idea that life began in an inferno, not a warm pond. "The evidence suggests that both the archaea and the bacteria are of thermophylic ancestry," he said. "And that makes a strong case that their common ancestor, which is the ancestor of all living things, was a thermophile."

Diagrams: "Life's Hot Incubator" The inferno of the early earth meant that the first life had to be heat-loving, scientists are concluding. The planet coalesced from cosmic debris, was pounded by swarms of huge rocks that probably carried enough energy to vaporize the seas and finally cooled down enough to support life, perhaps as early as four billion years ago. Today, hot spots on the earth's surface teem with the direct descendants of the first life. "Hiding Places of the Living Relics" A. Kyushu Island, Japan. B. North Slope oil reservoirs. C. Mount St. Helens. D. Yellowstone hot springs. E. Iceland hot springs. F. Mid-Atlantic Ridge. G. Vulcano, Italy. H. New Zealand hot springs. I. North Fiji Basin. J. East Pacific Rise (Source: University of Washington/Seattle, School of Oceanography) (pg. C1) "Tree of Life Found to Hide Hot Roots" The phylogenetic tree, left, shows the evolutionary relationships between different types of organisms and lists the maximum survival temperatures of some branches. In general the more ancient the origin of the organism, the higher the temperature at which it is found to thrive, suggesting that life on the planet arose in an inferno. (Source: Dr. Carl R. Woese/University of Illinois (tree); "Biology of Microorganisms," T.D. Brock, et al. (Prentice Hall) (pg. C7)